Electron linear accelerators with energies up to 50 MeV have been widely used for radiation therapy and industrial radiography since early 1960. Currently an emphasis is being placed on more efficient, compact, and cost-effective designs. For standing-wave, Pi/2 mode linear accelerators, the coupling cavities allow for a flexibility of design, since they are unexcited in steady state operation. Existing standing-wave accelerator coupling cavities can be placed in four general design types: on-axis, coaxial, side cavity, and annular ring structures. These four structures are shown schematically in FIG. 1.
Since the side cavity structures are off-axis, they do not influence the design of the accelerating cells, enabling side coupled accelerators to attain high efficiencies. Side coupled structures, however, have the disadvantages of increasing the effective diameter of the accelerator guide and the number of machining and assembly steps required.
Cylindrically symmetric cavities, the on-axis, coaxial, and annular ring designs, have the advantage of being machined directly into the opposite side of an accelerating cell, thereby eliminating multipiece assembly and prebrazing. Construction costs can be substantially reduced. Existing designs, however, all have disadvantages. The radius of an on-axis coupling cavity is comparable to the radius of the accelerating cavity. The structure is susceptible, however, to the excitation of parasitic and beam blowup modes, which reduce the overall accelerator efficiency and beam stability. (See J. P. Labrie and J. McKeown, "The Coaxial Coupled Linac Structure", Nuclear Instruments and Methods, No. 193, pp. 437-444, 1982). On-axis structures are also sensitive to thermal detuning, a result of the thermal deformation of the web between the accelerating cells. (See: J. McKeown and J. P. Labrie, "Heat Transfer, Thermal Stress Analysis and the Dynamic Behavior of High Power RF Structures", IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, pp. 3593-3595, 1983).
Coaxial structures eliminate the direct interaction of the electron beam with the coupling cavity, but designs of the prior art increase the effective guide diameter 60% to 80%. Prior art designs consist of narrow cylindrical cavities sandwiched between accelerating cells, which operate at a coaxial TM.sub.010 -like mode. (See for example: C. Fuhrmann et al, "Characteristiques de Dispersion et Impedances Shunt de Trois Structures Biperiodiques Acceleratrices en Bande S", Nuclear Instruments and Methods in Physics Research, No. 227, pp. 196-204, 1984 and R. M. Laszewski and R. A. Hoffswell, "Coaxial-Coupled Linac Structure for Low Gradient Applications", in Proceedings of the Linear Accelerator Conference 1984, pp. 177-179). Annular ring designs in the prior art have the same size disadvantage as the existing coaxial structures, along with increased machining complexity.